Electrochemical capacitors are a class of high-rate energy storage devices which use electrolytes and electrodes of various kinds in a system similar to that of conventional batteries. Electrochemical capacitors like batteries are essentially energy storage devices. However, unlike batteries, they rely on charge accumulation and/or charge transfer at the electrolyte/electrode interface to store energy. Charge storage in electrochemical capacitors is therefore substantially a surface phenomena since operating current is so high that only the surface of the electrode is employed. Conversely, charge storage in batteries is a bulk phenomena occurring within the bulk of the electrode material.
Electrochemical capacitors can generally be divided into one of two subcategories: double layer capacitors in which the interfacial capacitance at the electrode/electrolyte interface can be modeled as two parallel sheets of charge; and pseudocapacitor devices in which charge transferred between the electrolyte and the electrode occurs over a wide potential range, and is the result of primary, secondary, and tertiary oxidation/reduction reactions between the electrode and the electrolyte. These types of electrochemical capacitors are being developed for high pulse power applications.
Most of the known pseudocapacitor active materials are based on metallic elements such as platinum, iridium, ruthenium, or cobalt. These materials are generally quite expensive and pose a significant hurdle to the wide-spread commercialization of this technology. Moreover, the use of two electrodes fabricated of similar materials in a symmetric configuration and having redox potentials in a relatively narrow voltage range restricts the cell voltage and hence the deliverable energy density. That is, the voltage ranges are small and hence the commercial applicability of the device is limited. Other less expensive materials have been tried but have been less than successful. For example, workers in the field have attempted to fabricate devices using pressed powder cobalt and cobalt oxide electrodes. However, these types of electrodes have failed for numerous reasons including for example poor cycle life, and inability to achieve desired electrochemical performance characteristics.
Accordingly, there exists a need for an electrochemical capacitor electrode material which delivers good performance in terms of energy storage, power density and cycle life. Moreover, such material should be abundant in nature, readily processable into devices, and relatively benign environmentally.